This application claims priority to an application entitled xe2x80x9cOptical Fiber Preform Having OH Barrier and Fabrication Method Thereofxe2x80x9d filed in the Korean Industrial Property Office on Jan. 28, 1999 and assigned Serial No. 99-2696, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates generally to an optical fiber preform formed by MCVD (Modified Chemical Vapor Deposition), and in particular, to an optical fiber preform having an OH barrier and a fabrication method thereof.
2. Description of the Related Art
Due to the advantages of drawing a long optical fiber drawn from a unit preform, a preform should be formed with large diameter to increase the productivity of optical fiber. In fabrication of an optical, fiber preform by modified chemical vapor deposition, how thick a core layer can be deposited is a key issue to production of a large-diameter preform. In the case of a large-diameter preform, however, heat is not fully transferred to the core layer due to tube collapse and increased tube thickness during the deposition, resulting in bad sintering and consolidation of the core layer.
A single-mode optical fiber is formed by depositing a cladding layer and a core layer. For fabrication of an optical fiber preform for a DC-SM (Depressed Cladding-Single Mode) type, a cladding layer is formed by depositing SiO2 (silica) doped with P2O5, GeO2, and F to reduce deposition temperature and refractive index, a core layer through which light is transmitted is formed by depositing SiO2 doped with GeO2, and the deposited cladding layer and core layer are collapsed and closed.
In the process of fabricating an optical fiber preform by modified chemical vapor deposition, a tube self-collapses during deposition as a deposited layer becomes thicker and, as a result, the thickness of the deposited layer is further increased. A high-temperature burner is required to sinter and consolidate the thick deposited layer. The resulting long collapse and closing process leads to a long time exposure of a substrate tube to high temperature. Therefore, it is difficult to form a preform from which 300 km or longer optical fiber can be drawn.
If the preform is formed in such a way that the diameter ratio of the cladding layer to the core layer (b/a) is small, OH absorption loss is drastically increased. That is, a very small amount of moisture (generally, a few ppm) included in the substrate tube is introduced into the deposited layers and combined with SiO2 or P2O5 deposited in the cladding layer, producing a Pxe2x80x94Oxe2x80x94H or Sixe2x80x94Oxe2x80x94H bond. OH penetrated even to the core layer is combined with SiO2 or GeO2, releasing Sixe2x80x94O or Gexe2x80x94O bonds and producing Sixe2x80x94Oxe2x80x94H or Gexe2x80x94Oxe2x80x94H bonds, instead.
The above Oxe2x80x94H or Pxe2x80x94Oxe2x80x94H bond adds to optical loss caused by an absorption band of a specific wavelength region. In the case of a single-mode optical fiber, the Oxe2x80x94H bond significantly influences optical loss at wavelengths of 1.24 and 1.385 xcexcm and the Pxe2x80x94Oxe2x80x94H bond in a wavelength region ranging from 1.2 to 1.8 xcexcm. OH introduced into the core area forms a non-bridging oxygen (NBO). The resulting density fluctuation in the core layer increases scattering loss.
In addition, as a deposited layer becomes thicker, the inner and outer diameters of a tube decrease during sintering and consolidation simultaneous with deposition. Therefore, it is difficult to obtain an optimal diameter ratio (cladding diameter/core diameter=b/a) and thus have a thickness of a layer enough to prevent OH diffusion, resulting in a great increase of OH-caused loss.
A cladding layer may be formed to be thick to prevent penetration of OH from a substrate tube into a core layer in prior art. In fabricating a large-diameter preform using this method, however, tube contraction makes it difficult to obtain an optimal diameter ratio and the increase of tube layer thickness during deposition of a core layer reduces a heat transfer efficiency. Thus, a higher temperature burner is used and long exposure of the tube to high temperature further increases OH-caused loss.
Examples of optical fibers and preforms of the conventional art are seen in the following U.S. Patents. U.S. Pat. No. 4,114,980, to Asam et al., entitled Low Loss Multilayer Optical Fiber, describes an optical fiber made from a deposited silica tube. A barrier layer is interposed between the silica tube and the cladding layer to prevent migration of OH.
U.S. Pat. No. 4,385,802, to Blaszyk et al., entitled Long Wavelength, Low-Loss Optical Waveguide, describes an optical fiber having a core, a first inner cladding layer having P2O5, and a second inner cladding layer disposed between the first inner cladding layer and the core to prevent P2O5 from diffusing into the core.
U.S. Pat. No. 4,447,127, to Cohen et al., entitled Low Loss Single Mode Fiber, describes a double clad optical fiber.
U.S. Pat. No. 5,090,979, to Le Sergent et al., entitled Method of Manufacturing An Optical Fiber Preform Having Doped Cladding, describes a preform for an optical fiber. The preform has a support layer, a substrate layer, a core and a cladding.
U.S. Pat. No. 5,838,866 to Antos et al., entitled Optical Fiber Resistant To Hydrogen-Induced Attenuation, describes an optical fiber with a central core, an inner cladding region containing germanium dioxide, and an outer cladding region.
U.S. Pat. No. 5,942,296, to Oh et al., entitled Optical Fiber Preform, describes an optical fiber preform made from a first quartz tube used as a clad, having a deposited layer and a clad layer, and a second quartz tube jacketing the first quartz tube. Use of the first quartz tube reduces the OH concentration.
However, the inventions described in these patents do not solve the above-mentioned problems.
It is therefore an object of the present invention to provide an improved optical fiber preform.
It is also an object of the present invention to provide an improved method for manufacturing an optical fiber preform.
A further object of the invention is to provide a larger optical fiber preform.
A yet further object of the invention is to provide an optical fiber preform from which more than 300 km of optical fiber can be drawn.
A still further object of the invention is to provide an optical fiber preform and a fabrication method thereof, in which the refractive index distribution of a single-mode optical fiber drawn from the fiber is improved.
Another object of the invention is to provide an optical fiber preform yielding an optical fiber having a low diameter ratio.
Still another object of the invention is to provide an optical fiber preform yielding an optical fiber having low optical loss caused by hydroxyl.
The above objects are achieved by providing an optical fiber preform. The optical fiber preform includes a substrate tube, a cladding layer, a core layer with a refractive index greater than the refractive index of the cladding layer and having an increased value toward the center thereof, and a first barrier formed between the substrate tube and the cladding layer by depositing a material with a low OH diffusion coefficient, for preventing OH included in the substrate tube from penetrating into the cladding layer.